Excavators, power shovels and similar earth-moving equipment are typically equipped with a swing drive that rotates the upper carriage (upper machine structure including the working tool) with respect to the undercarriage (lower machine structure with tracks or wheels for propulsion). The swing drive may be powered by hydraulic or electric motors. Swing speed control may be utilized on construction excavators, backhoes and similar machines. That is, when the operator moves a control lever, the position of the lever corresponds to a desired rotational velocity of the swing drive. The operator may adjust the lever command to obtain the desired speed and to compensate for changes in payload, linkage position or other factors that may affect swing speed. Large inertial loads, such as are common with large cranes or mining shovels, may be controlled by swing torque control. Swing torque control means that the operator lever position is interpreted as a desired motor torque, allowing the operator to modulate both the speed and acceleration of the swing drive.
For hydraulically powered swing drives, swing control has historically been accomplished using hydro-mechanical valves in the hydraulic circuit and the swing control characteristics (swing speed control, swing torque control) of such swing drives are primarily determined by the selection and setting of flow and pressure control valves, pump displacement control mechanisms, and other hydromechanical components. In other words, whether a hydraulic swing circuit primarily uses speed or torque control is determined by the hydraulic hardware because such hydraulic swing circuits primarily provide torque control or speed control for the swing motor but do not provide the option to have either torque control or speed control with the same hydraulic circuit.
Due to the size and weight of the upper carriage, there are large inertial forces to be overcome during initial movement. Displacement control pumps are not used to control speed of the swing motor because the swing speed tends to oscillate due to the amount of fluid pressure required to initiate movement, the large compressible volume hoses between the pump and swing motor, and the lack of any significant oscillation damping benefit provided by a work surface (ground, mine wall, etc.) in resistive contact with the upper carriage (the upper carriage swings through the air). Moreover, performance with a displacement control pump may be further decreased if a closed loop hydraulic circuit is utilized.
U.S. Pat. No. 6,520,731 (“MacLeod”) issued Feb. 18, 2003 describes a control system for swing cylinders to position a boom on a backhoe. The system includes a pair of double acting hydraulic cylinders on a backhoe frame operatively connected to the boom for swinging the boom with respect to the frame, a pump arranged in a closed circuit with the hydraulic cylinders such that the control of the pump is the sole means of controlling the cylinders. The disclosure does not address controlling bouncing/oscillating between decreasing and increasing signals for fluid volume displacement. A better design is needed.